Can you get on them with a pair of visegrips and twist and pull them out? If they won't wiggle back and forth and pull out maybe try unscreing them anyway. Maybe they thread and jsut don't have flats or have 2 flats on the rounded parts?

Beware if you have real short base fittings like used on thin tubes or such with shafts thru them, that there may Not be enough metal there to use a screw-in type. It will hit the shaft inside and cause binding or lock it up. You would have to take a 1/4' nut and weld on to the outer tube to use standard grease fitting there. Not sure how they got them in when new, but I assume it comes installed on a new part if one was still available.